This chapter contains sections titled: Self-diffusion, tracer diffusion and tracer impurity diffusion Nonsteady-state diffusion Steady-state diffusion Temperature variation of the diffusion coefficient The effect of impurities The penetration depth Self-diffusion mechanisms Atomic movement during diffusion Atomic migration and diffusion coefficients Self-diffusion in crystals The Arrhenius equation and the effect of temperature Correlation factors for self-diffusion Ionic conductivity The relationship between ionic conductivity and the diffusion coefficient Answers to introductory questions Further reading Problems and exercises